If you want to find out where that
X-UA-Compatible:IE=7 response header is coming from, there are several plugins for firefox you might find useful. I'm not sure of the names (I forgot, I switched to Opera and Chromium a long time ago) but search for 'Response Hearders' and for 'User Agent'... The first search should show you a choice of plugins that allow you to visualise the servers response headers (obviously) and the second search will get you plugins that allow you to make FireFox identify itself as another browser.
Use the User Agent switcher to identify as IE 8 and then revisit your site, checking the response headers. You'll see the one I am talking about as last one.
Then switch off ALL plugins and revisit your site again in the same way, checking again if the response header is still there. If not, re-enable half of the plugins you had disabled and check again. Go on until you know exactly which plugin is causing the response header and then try to find out why...
But if the response header did not go away after disabling all plugins then switch to Twenty Twelve (or Eleven) theme and check again.
If it's still there, I would be very surprised because it is not your Apache server that sends this header (I've checked by visiting a static html file on your site)... Create a fresh new PHP file called test.php with just one line of content:
<?php echo 'test'; ?>
Upload it to your site root, visit it the same way as before and check the response headers again.
Make sure to have your browser identify as IE 8 each time otherwise your results will get very confusing ;)